Interesting little site that allows students to draw, create scenes, and even make their own little book. The various activities are labelled according to age appropriateness, though it may not be realistic for the upper grades. It is possible that pieces such as "Tell a Story" could appeal to some 8th grade students, but overall the look and feel of the site is more suitable for elementary students.
This site also provides lesson plans that explain how to implement the activities in the classroom. These lesson plans include examples that fall into math, science, and language arts.
This site provides collaboration, business, and productivity tools. Specifically, it mentions speadsheet and data collection tools which could be used to address standards within math, science, and language arts. It does require an account, and it is free to sign-up. This site is more appropriate for high school, though the tools could be implemented in the middle grades. It does not seem to provide anything that is not already available through Google, but it does provide another choice.
Iknowthat provides supplemental resources for K-6. It boasts that it can be used to provide "extra practice" in all contents. For example, students can play games that help with parts of speech and vocabulary development. A very easy to use site that can provide that extra push for students.